In vitro transcription of molecularly cloned murine retroviruses has been examined. Two type C retroviruses, AKR murine leukemia virus and Harvey sarcoma virus, were found to contain active RNA polymerase II promoters within the viral long terminal repeat sequences located at either end of the virus. Therefore, viral promoters can initiate transcription of adjacent cell sequences in vitro. In addition to the promoter within the LTR, Ha-MuSV contains a RNA polymerase III promoter. The 5' end of this transcript is approximately coincident with the viral transforming gene. This transcript has also been identified in Ha-MuSV transformed NIH 3T3 cells. Unlike type C retrovirus, the LTR promoter of the type B retrovirus. mouse mammary tumor virus, is inactive in our cell-free transcription system. Since initiation of MMTV transcription is regulated by glucocorticoid steroid hormones, it is likely the presence of hormone-receptor complex, as well as chromatin components that interact with this complex, are necessary for in vitro transcription of MMTV. Experiments designed to test this possibility and also to pinpoint the hormone target site within MMTV are in progress.